This invention relates to a tin, lead, or tin-lead alloy plating bath. It particularly relates to a tin, lead, or tin-lead alloy plating bath which can be used in a substantially neutral pH range (pH 2.0-9.0), characterized by the addition of an alkali metal salt of an aliphatic or aromatic sulfocarboxylic acid.
Tin plating and tin-lead alloy plating have in recent years been widely used in light electric and electronic industries to form coatings for enhanced solderability or as etching resists on their component parts. The plating techniques have, however, left much room for improvement. The electronic parts that integrally incorporate elements of insulating materials such as ceramics, lead glass, or plastics and electroplated members, call for plating capable of ensuring excellent solder-ability and adhesion without any such drawback as corrosion, deformation, or change in properties of the products.
For the plating of the electronic parts of the character it has been routine to use a bath of borofluoride, sulfuric acid, organic sulfonic acid or the like. A typical example of the organic sulfonic acid bath is disclosed in Japanese Patent Application Publication No. 16176/1974. The specification describes an electroplating bath containing a complex salt of sulfonic acid by subjecting an excess amount of an aliphatic or aromatic sulfonic acid to the action of a compound of the metal to be electrodeposited. Such a plating bath, containing a large proportion of free sulfonic acid, is strongly acidic with pH 1.0 or below. The borofluoride and sulfuric acid baths too are strongly acidic. A disadvantage common to these baths of high acidity is, for example, the attack on lead glass in the course of tin-lead alloy plating of integrated-circuit parts that use the particular glass.
To eliminate this disadvantage, it was attempted to adjust the pH of the plating bath so as to be close to neutrality. However, the tin ions, normally stable in an acidic bath, form a white precipitate of stannous hydroxide at pH values around neutrality, rendering tin or tin-lead alloy plating infeasible. If the formation of such a white stannate precipitate is to be avoided, the addition of a complexing agent, e.g., gluconic, citric, tartaric, or malonic acid, will be necessary. Such a complexing agent tends to decompose on electrolysis or reduce the current efficiency, and makes it difficult, especially in tin-lead alloy plating, to control the electrodeposit composition (Sn/Pb).
It has now been found, after our search for ways of overcoming the drawbacks of the strongly acidic plating baths, that the addition of a certain aliphatic or aromatic sulfocarboxylic acid as a complexing agent stabilizes a tin, lead, or tin-lead alloy plating bath without the formation of a stannous hydroxide precipitate, even in the pH range around neutrality.